Natural products are a rich source of pharmaceutically active compounds. One class of natural products are produced by the non-ribosomal peptide synthetases (NRPSs) through a fascinating modular, multidomain architecture. During synthesis, the peptide intermediates are bound to an integrated carrier protein that is delivered in a coordinated manner to neighboring catalytic domains. A complete understanding of the NRPS structural enzymology, including the active sites and the domain interfaces, would allow the identification of novel products both through analysis and prediction of uncharacterized biosynthetic clusters and the engineering of novel chemical moieties into previously identified NRPSs. To continue our structural interrogation of NRPSs, we will determine structures of NRPS multidomain enzymes, including full-length modules, with informative ligands. Our studies will address multiple NRPS systems that perform standard and unusual peptide syntheses. We will additionally examine the structural basis of a family of small auxiliary proteins that enhance NRPS adenylation domains. And finally, we will examine cytotoxin production from a novel NRPS system from a human commensal bacteria. These studies will use biochemical, biophysical, structural, and chemical techniques to continue our work to provide a complete structural description of the active sites and the domain choreography of the modular NRPS enzymes.